The present invention relates, in general, to printed circuit board testing, and, more particularly, to the testing of bare printed circuit boards by the use of an electrolyte.
Printed circuit boards, or xe2x80x9cPCBs,xe2x80x9d have been used in the electronics industry for many years as a valuable means of connecting many separate electronic components, such as integrated circuits (xe2x80x9cICsxe2x80x9d), resistors, capacitors, and others, to form an electronic circuit necessary to perform a desired function.
Conductive connection points on the boards, called xe2x80x9cpads,xe2x80x9d are the areas of the board to which the various electronic components are connected. The pads are, in turn, connected to each other with printed wiring to form xe2x80x9cnetsxe2x80x9d so that there are essentially zero ohms of impedance between any two pads on the net. Conversely, two pads on different nets essentially have an infinite amount of impedance between them.
To further define some terminology, a xe2x80x9clayer,xe2x80x9d as it relates to printed circuit boards, is a collection of pads and associated printed wiring that substantially reside within the same geometric plane. A PCB has one or more such layers, with each layer being isolated from the next by a stratum of nonconductive material. Many PCBs have two or more layers, and some will have several, with only the outermost layer on each side of the board having pads. Some of the layers may even be power or ground distribution xe2x80x9cplanes,xe2x80x9d with those layers consisting mostly of metal, as opposed to a small amount of printed wiring. Portions of a net may reside on several different layers, with xe2x80x9cvias,xe2x80x9d or plated-through holes, connecting those portions to form the net.
For descriptive purposes, FIG. 1 presents an edge view of an example of a 2-layer printed circuit board 1, and FIG. 2 provides a plan view of the top of board 1. The conductive portion of board 1 consists primarily of layers 7 and 8, with the layers being separated by nonconductive material 9. Printed wires 2 connect solder pads 3 to each other to form nets 4. Via 5 is utilized on this board to connect two portions of a net 4 that are on opposite sides of board 1. For purposes of illustration, the height of layers 7 and 8, as shown in FIG. 1, is greatly exaggerated compared to the thickness of nonconductive material 9.
Prior to attaching electronic components to the PCB, a solder xe2x80x9cmask,xe2x80x9d or protective layer, is usually applied to the two sides of the board. As shown in FIG. 3, solder mask 6 covers printed wiring 2 (FIG. 1), vias 5 (FIG. 1), and other areas of the board, essentially leaving only connective pads 3 exposed. Solder mask 6 serves to prevent solder from adhering to unwanted areas of the board, as well as to protect the board from damage due to abrasion or corrosion.
In order to guarantee a certain level of quality in the products that incorporate such boards, the PCBs are usually tested alone prior to being assembled with the rest of the product in which they reside. At least some of this testing is performed after the boards have been populated with electronic components.
Over the years, printed circuit board technology has progressed. More specifically, the number of layers allowed has increased, while the distance between printed wires, and the width of the wires themselves, have decreased. As a result, the number of components that can be attached to a given amount of surface area on a board has increased substantially. At the same time, the electronic components that populate these boards have become more complex, due to advances in IC technology. Therefore, it is now common for the overall cost of a board populated with electronic components to be determined primarily by the cost of the individual components rather than by the cost of the xe2x80x9cbare,xe2x80x9d or unpopulated, board.
A direct result of these advances is that testing the bare board before attaching the electronic components is essential, since the discovery of a board defect after populating the board may now result in a much greater cost than if the defect were found prior to attaching the components. The two most common defects are xe2x80x9cshort circuits,xe2x80x9d whereby two nets that are intended to be separate are, in fact, connected, and xe2x80x9copen circuits,xe2x80x9d whereby a portion of a net is inadvertently disconnected from the remainder of the net. In the case that such defects are discovered during testing, the bare board is typically discarded, since repairing the board is generally not economically feasible.
Currently, two basic methods exist for testing bare boards. In the first case, a test fixture is constructed which contains a contact probe for each pad on the board. For boards with pads on each side (called xe2x80x9ctwo-sided boardsxe2x80x9d), the fixture has two sides and is capable of contacting all of the pads on both sides. Each contact probe is connected in some fashion to a test algorithm controller, which checks for wanted or unwanted electrical continuity between any two probes, thereby checking for any of the short circuit or open circuit board defects mentioned above. Tests can be performed quickly using such a system, but a new test fixture must be manufactured for each board design to be tested. Any changes in the board require commensurate changes in the test fixture. Also, test fixtures for the more complex board designs may require tens of thousands of probes, making such fixtures expensive, fragile, and difficult to modify.
The second method involves a xe2x80x9cflying probexe2x80x9d technique, whereby two or more probes-are mechanically maneuvered over, and possibly under, the board under test by a probe movement mechanism. Two or more pads are contacted at a time, and a continuity measurement is made. Due to the need to move the probes horizontally over the board for each measurement, this method will be inherently slower that the first method described. On the other hand, using the flying probe method is generally much less expensive, because little xe2x80x9cfixturing,xe2x80x9d or modification of the tester specific to the board under test, is required, and design changes to the board under test are easily accommodated by changes to the test algorithm controller software.
One major drawback of both techniques discussed above is that the continuity of each pad is tested with respect to every other pad on the board to guarantee that no unintended short circuits or open circuits exist. Testing of each pad on each net in this manner is typically a time-consuming and relatively expensive process. Therefore, a need currently exists for a method of bare board testing that can measure each net without requiring the probing of each pad on a net, or pads on other, unrelated nets.
In the embodiments discussed below, the invention provides a way to test a bare printed circuit board without probing every possible pad combination, saving both time and expense during testing. The associated test fixture necessary to implement an embodiment of the invention is simple to manufacture, and easy to modify in response to board design changes.
One embodiment of the bare board tester according to the invention includes a conductive tank, together with a way to support the bare board within the tank. Contained within the tank is a sufficient amount of an electrolyte to immerse the board. One or more probes are then used to contact the pads of the board. The probes are used to measure the electrical resistance between the probe and the conductive tank. In some embodiments, this measurement is performed by applying a voltage across one of the probes and the tank, thereby generating an electrical current through the pad being contacted by the probe, the net to which the probe is attached, and the electrolyte. The amount of current is indicative of the resistance encountered, which, in turn, indicates the total surface area of that net that is exposed to the electrolyte. If the printed circuit board includes a solder mask that leaves only the connection pads exposed, and the pads are all essentially the same size, then the measured resistance is directly related to the number of pads connected to the net being tested. In such an embodiment, a means to measure this current is provided to determine if the net is connected to the proper number of exposed pads. therefore, only one pad per net is required to be in contact with a probe to determine if the corresponding net is intact.
In an example embodiment of the tester according to the present invention, the probing mechanism used is one or more X-Y flying probes, moved about the board, and contacting one pad at a time from each net in sequence. In another embodiment, the probe is one of several fixture probes, whereby one probe for each net exists. The fixture probes are thus stationary, with one probe being used at a time in sequence to perform the test on each net.
In yet another embodiment, the tester includes a method for preventing the formation of gas bubbles in the electrolyte, which would tend to reduce the surface area of the pads that are in contact with the electrolyte. For example, the electrolyte may be stirred or agitated to serve that purpose.
The invention also is embodied in a method of immersing a bare printed circuit board in an electrolyte and measuring the resistance across a net under test and the electrolyte to detect the presence of unintended short or open circuits. Defective nets can be detected by comparing the resistance value associated with a net with a predetermined theoretical resistance value for that same net. Alternately, that decision can be made by comparing the measured resistance related to each net with that of other nets, since the ratio of any two measured resistances will be consistent regardless of factors such as ionic concentration in the electrolyte and the resistance measurement circuitry used.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.